Several carburetors have been proposed for the purpose of creating an air-fuel mixture of substantially constant (usually stoichiometric) air-fuel ratio for an internal combustion engine. In general, it has been contemplated that such a carburetor would be used in a closed loop system having a sensor which, for example, measures the oxygen content of the engine exhaust gases as an indication of the air-fuel ratio of the mixture created for the engine and which initiates a feed-back signal causing the carburetor to create a mixture of the desired air-fuel ratio.
Certain carburetors adapted for that application have an electronically controlled device which drives a fuel metering apparatus between rich and lean positions according to a pulse width modulated duty cycle; the metering apparatus is maintained in the lean position for a portion of the duty cycle and in the rich position for the remainder of the duty cycle. The carburetor thus duty cycle modulates the fuel flow between a maximum and a minimum and then averages the maximum and minimum fuel flows to create a mixture of the desired air-fuel ratio.